1. Field of the Invention
The present invention relates to a composition and method for treating contact lenses. More particularly, the present invention relates to a composition and method for soaking and sterilizing soft contact lenses.
2. Background of the Prior Art
Hydrophilic or partially hydrophilic plastic materials have been described for use in making so-called soft contact lenses. For example, U.S. Pat. No. 3,503,393 to Seiderman and U.S. Pat. No. 2,976,576 to Wichterle describes processes for producing three-dimensional hydrophilic polymers of polyhydroxyethylmethacrylate in aqueous reaction media having a sparingly cross-linked polymeric hydrogel structure and having the appearance of elastic, soft, transparent hydrogels. Other soft contact lenses include lenses made out of silicone and other optically suitable flexible materials.
The main virtues of these lenses is their softness and optical suitability. The hydrophilic lenses are particularly useful in ophthalmology due to their remarkable ability to absorb water with a concomitant swelling to a soft mass of extremely good mechanical strength, complete transparency and the ability to retain shape and dimensions when equilibrated in a given fluid.
One of the problems connected with these soft contact lenses is the method of their sterilization and cleaning. The very property of the hydrophilic soft lenses which allows them to absorb up to 150 percent by weight of water also allows preservatives which might otherwise be used for cleaning and sterilization to be absorbed and even concentrated and later released when the soft contact lens is on the eye. The release may be much slower than the uptake, thereby allowing the preservative to build up in the lenses. This build-up eventually effects the physical characteristics of the lenses including dimension, color, etc. This can have the harmful result of damaging or staining the contact lens itself and/or harming the sensitive tissues of the conjunctivae or cornea.
Hard contact lenses do not absorb appreciable amounts of water (i.e. 0.1-0.4%) and thus the use of effective preservatives does not create a problem in the hard contact lens field. However, as stated in U.S. Pat. No. 3,689,673, sterilization of hydrophilic soft contact lenses may be carried out by first soaking in hydrogen peroxide, then sodium bicarbonate and finally normal saline, all at room temperature or by boiling the lenses in normal saline. Furthermore, users of soft contact lenses are warned that under no circumstances should solutions designed for hard contact lenses be used, for the reason that the preservatives in such solutions will be absorbed and even concentrated by the soft lens and may seriously damage the soft lens and/or the eye of the user.
U.S. Pat. No. 3,689,673 further discloses a process of soaking and sterilizing hydrophilic soft contact lenses. Therein it is disclosed that a number of commonly used antimicrobial agents are concentrated in the soft lens. The patent also suggests that these materials may cause corneal damage and that similar in vitro and in vivo tests have shown the undesirability of such antimicrobial agents when used with hydrophilic lenses.
A quaternary ammonium compound having the formula ##STR1## wherein R is tallow is available from the Miranol Chemical Company under the tradename MIRAMINE.RTM. TA-30. This compound may be described chemically as a quaternary ammonium compound and has been described as a hair and skin conditioner.
Quaternary amines, e.g. benzalkonium chloride (BAK), are generally known to be bactericidal and are non-toxic to the eye at levels below about 0.01 %. As a result, BAK is a commonly used preservative in ophthalmic preparations and hard contact lens care products. However, the same levels of BAK are toxic to the eye when used with soft contact lens because of the tendency of BAK and other quaternary amines to concentrate in the soft lens.
It would therefore be desirable to discover a sterilizing solution for soft contact lenses which does not concentrate in the soft lens, which is non-toxic to the eye and which does not damage or discolor or change the shape of the soft lens.